Binocular Disparity

Notice that the tree and the policeman are about the same size on the retina. One would undoubtedly perceive the tree to be further away than the policeman. There are multiple reasons for this. One, of course, is that most trees, this well developed, are much larger than people. Consequently knowing that the size of the tree is likely to be much larger than the policeman helps to make it appear further away.

There is another factor that helps with the perception of distance and that is binocular disparity. In the disparity diagram the eyes are fixated on the tree. The red lines between the tree and the retinas describe the angle of convergence the eyes make when the tree is imaged on both foveae. It is said that the tree is imaged on corresponding points of the retina and thus there is no disparity.

By corresponding points I mean those points that would be identical if one retina was moved over to superimpose the other retina.
Because the eye are fixated on the tree, the policeman, who is closer to the observer, has its image falling on non-corresponding points of the retinas. That is to say its images are binocular disparate. This binocular disparity provides an additional cue to the distance of the policeman relative to the distance of the tree.

As an aside, if images are too disparate on the retina one will perceive two images instead of one (double vision). You can create this for yourself by holding your two pointing fingers in front of your face. Hold one so that it is behind the other. Now fixate on the finger in the front. You will notice that if the second finger is no too far behind the first it will appear as a single finger. But as you move the second finger further way from the first it will appear double. Now fixate the second finger while it is a this distant position. Note, that now it appears singular but the finger closer to your face now appears double. The single appear finger is on corresponding points of your retinas the one that appears double is not. The visual system can tolerate a certain amount of disparity without causing double vision. As long as two images lie within what is called Panum's area disparate images will not appear double.